The overall objective of this research effort is to study the possible physiological and pathophysiological roles of intracellular Na ion and K ion, making use of recent technological advances in constructing ion-selective microelectrodes. The primary objective will be to relate changes in the electrochemical activities of Na ion and K ion within the cellular cytosol to transepithelial transport. The secondary objectives will be to: (1) examine the potential roles of intracellular Na ion and K ion in the development of malignant transformation, and (2) define the role of subcellular compartmentalization in regulating the cytoplasmic activities of Na ion and K ion.